The invention relates to a radiation detection device with high dynamic range. By high dynamic range detection device one means a device capable of detecting both low flux and high flux radiation.
The radiation in question can be X or xcex3 radiation, but one can also use other types of radiation, of corpuscular type for example, such as proton beams. The only restriction is that this radiation must be able to create electric charges within a semiconducting material in a volume of the order of a cubic millimetre.
The invention can be applied in particular in the medical domain. For example, for radiography, the X-rays used, before arriving on the detector device, cross the body of a patient or are absorbed in non-homogeneous fashion. The exiting flux can thus vary considerably locally (several decades).
Another example can be taken from radiotherapy: the patient can be irradiated at very low level to begin with for positioning, and then very highly irradiated afterwards for treatment.
A further domain is that of non-destructive monitoring by radiography, for example inside containers (loading ships) which can have very varied absorption levels.
At present, and in particular in the medical domain, radiation detectors (for example X or xcex3) are usually scintillation detectors operating on a principal of indirect detection: the incident photon reacts with the scintillation substance, creating photons of another type, photons which are multiplied by a photo-multiplier in order to provide a measurable electric signal.
These detectors have an efficiency and a resolution which can be insufficient for certain applications.
These characteristics can be improved by replacing the scintillation detectors by semiconductor detectors. FIG. 1 is a diagram of a detector of this type.
As shown, the detector comprises a semiconducting material 2 sandwiched by two electrodes 4 and 6, supply means 8 capable of bringing the electrode 6 to an appropriate voltage (xe2x88x92V), means for measuring the current (i) delivered, comprising, in the example shown, an amplifier 10 whose output is returned to the input by a condenser 12, a circuit-breaker 14 also being attached to the terminals of this condenser. The device also comprises an apparatus for measuring the current (or the voltage 16). The radiation one wishes to detect is referenced 20 and crosses the semiconducting material 2.
The operation of this device is as follows. The radiation 20 interacts with the semiconducting material 2, creating electric charges. For the same incident radiation, the number of charges created is of an order of size greater than that obtained by indirect detection by a scintillation detector. The electric field created in the material by the electrodes makes it possible to collect these charges on the electrodes and in particular on electrode 4. These charges are then stored in the condenser 12 and processed in the circuit 16 which delivers a signal representative of the radiation.
The integration condenser 12 has dimensions enabling it to store the maximum quantity of charges that the semiconducting material can deliver. This quantity is a function of the value of the incident flux. If the flux is very high, the number of charges to store will be high and the capacities needed for storing them will he high. These capacity values may either not be available on the market for electronic components, or may involve integrated capacity surfaces that are too big for the space available on the circuit.
Thus the solution lies in reducing the quantity of charges to be stored, only when the incident flux is too high. But it is essential to preserve the fundamental qualities of the detector, that is to say good spatial resolution or good contrast. The spatial resolution is the minimum distance separating the points of interaction of two incident photons with the material in order that the detector can differentiate between them.
The quantity of charges to be stored cannot be diminished by simple reduction of the number of photons arriving at the detector. This method, valid for high fluxes, is not suitable for low fluxes and the detector would lose the dynamic range needed for the application. Furthermore, the number of incident photons must remain consistent since the noise of the electric signal is proportional to 1N where N is the number of photons absorbed by the whole volume of the detector.
The present invention has the particular aim of proposing a radiation detection device with high dynamic range, which does not have these drawbacks and makes it possible to satisfy these contradictory requirements.
In order to do this, the invention proposes a device whose essential characteristic is that the polarisation electrode, unlike the measurement electrode, is fragmented into conductive zones insulated from each other electrically, the supply means being capable of bringing each of these zones to an appropriate voltage.
During operation, the incident radiation is injected in a direction, perpendicular to the fractionation direction of the polarisation electrode.
These means make it possible to operate the device in two different ways, according to the applications envisaged:
either when working at low voltage, on the totality of the material, in which case low noise is obtained and the contrast is improved,
or when working on only a part of the material but at high voltage, in which case the spatial resolution is improved.
To be precise, the aim of invention is thus a device for detection of energy radiation, comprising a semiconducting material able to convert this radiation into electric charges, a measurement electrode and a measurement circuit for measuring the current delivered by this electrode, characterised in that it further comprises polarisation electrodes constituted of conductive zones insulated from each other electrically, said polarisation electrodes and the measurement electrode sandwiching the material and the supply means capable of bringing each of these conductive zones to an adjustable appropriate voltage.
Preferably, the semiconducting material takes the form of a parallelepiped bar with a depth intended to be oriented parallel to the direction of the radiation, a width and a height, this bar having two parallel faces separated by said height, the measurement electrode and the polarisation electrodes being set on these faces.
Preferably, in this variant, the conductive zones of the polarisation electrode are in the form of rectangular strips with a length parallel to the width of the bar and a width parallel to the depth of the bar.
In another variant, the measurement electrode is constituted of rectangular conductive strips with a length parallel to the depth of the bar and a width parallel to the width of the bar.
The fragmentation zones can take various forms, rectangular strips in particular.